[Workers] Add Error 501 Not Implemented documentation

[krys-cf]

Summary

[SPM-3086] Add documentation for HTTP Error 501 (Not Implemented) to the Cloudflare 5xx errors section.

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[elithrar]

/bonk review and suggest any fixes

[ask-bonk]

Overall this is a clean, well-structured addition that follows the existing patterns for 5xx error pages. A few things to address:

1. Missing trailing newline — the file ends without a final newline character, which will cause the No newline at end of file diff marker. Most editors and linters expect a trailing newline.
2. Style guide: e.g. → for example — the Cloudflare style guide explicitly says "Do not use i.e. or e.g. Instead, use that is or for example, respectively." The rest of the 5xx error pages consistently use "for example" rather than "e.g.".
3. Scope accuracy — the description says Cloudflare returns 501 when a request uses an unsupported HTTP method on the Workers Runtime. It may be worth confirming whether this applies only to Workers routes or also to other Cloudflare proxy scenarios. If it is truly Workers-specific, consider noting that in the page (similar to how error-500.mdx has a "Workers-specific causes" section).

[ask-bonk]

Review submitted. Here's a summary of the feedback I left on the PR:

4 inline comments posted:

1. Missing trailing newline (line 18) — The file lacks a final newline character. Suggested fix provided.

2. Style guide violation: e.g. → for example (line 14) — The Cloudflare style guide explicitly prohibits Latin abbreviations like e.g. in favor of "for example". All other 5xx error pages in this directory use "for example". Suggestion provided.

3. Minor: etc. in method list (line 13) — Optional suggestion to link to RFC 9110 instead of using an imprecise etc. trailing list, though the current wording is acceptable.

4. Scope question (line 9) — Asked the author to confirm whether the 501 response is exclusively from the Workers Runtime or if Cloudflare's proxy layer can also return it in other scenarios. If it's Workers-only, the page structure could better reflect that (similar to how error-500.mdx separates general vs. Workers-specific causes).

github run